This patch makes BUILD_MAP more like BUILD_LIST, and uses these newfound abilities to compile literal dictionaries into code more like that for lists and tuples.
No reason, really, beyong asthetics & the fact that this'll make some hacks I want to inflict on the function call mechanism easier...

While this change make the generated code a lot simpler, it
also has a high cost in speed and memory. The stack for a code
object needs to be big enough to hold all the keys and values,
which can be costly. The pybench dict creation benchmark slows
down about 15%.